Iron distributions in the equatorial Pacific: Implications for new production

Several recent studies have shown that phytoplankton growth rate and production at 0”, 14O”W is physiologically limited by iron. Therefore, changes in iron supply to the euphotic zone will result in variations in phytoplankton growth. We show that the flux of iron to this region is dominated by upwelling of the iron-rich Equatorial Undercurrent waters. Variations in the depth and strength of upwelling and changes in iron concentrations at the base of the euphotic zone will account for variations in primary and new production in this region. We determined dissolved and particulate iron profiles for the upper water column of the eastern equatorial Pacific including a vertical section from 9”N to 3”s along 14O”W. One of the more prominent features of the section was a peak in dissolved and particulate iron associated with the Equatorial Undercurrent. The possible lithogenic origin of this iron is substantiated by the vertical section of particulate aluminum and manganese, which is consistent with a shallow hydrothermal source in the western equatorial Pacific. A simple one-dimensional model was used to calculate iron fluxes into the euphotic zone at the equator. Upwelling rates and dissolved iron concentrations were coupled to estimate the upwelling iron flux at 120 m (0.1% light level). Diffusive and atmospheric inputs of iron were also considered but were less significant than the upwelling flux. Iron-based potential new production was estimated to be lo-82 mmol C rnd2 d-l with C : Fe ratios of lOO,OOO500,000: 1. In a similar manner, nitrate-based potential new production was 99-106 mmol C m-2 d-l. This demonstrates that iron supply limits new production to only 9-83% of the nitrate-based potential. The concept of new production as described by Dugdale and Goering (1967) is based on the idea that the input of upwelled and incorporated new nitrate is quantitatively equivalent, at steady state, to the export of biologically incorporated nitrogen. An increase in biomass or biological export must be balanced by an identical flux of nitrate into the euphotic zone. As much as 20% of the ocean surface area contains high levels of nitrate in the euphotic zone but relatively low amounts of chlorophyll (Martin 1990). In these areas the nature of phytoplankton limitation is currently under debate (Chisolm and Morel 1991). Recently, it has been shown that availability of iron can limit production in these areas (Martin and Fitzwater 1988; Martin et al. 1989, 1994). Additionally, Price et al. (1994) have concluded that iron ultimately regulates new production in the equatorial Pacific Ocean. Therefore, the supply of new iron to the euphotic zone may be a more relevant control of new production in these areas than upwelled nitrate.